1. Field of the Invention
The present invention relates to a control apparatus for a hydraulically operated vehicular transmission which has a plurality of hydraulic engaging elements.
2. Description of Related Art
In a hydraulically operated transmission for use in a vehicle such as an automobile, it is normal practice to effect the speed changing or shifting by controlling the engaging or disengaging of a plurality of hydraulic engaging elements. In this case, unless an appropriate control is made of a simultaneous engagement of a hydraulic engaging element to be engaged and a hydraulic engaging element to be disengaged at the time of speed changing, there will occur speed-change shocks (i.e., shocks at the time of speed shifting). As a solution, the following arrangement is disclosed in Japanese Published Unexamined Patent Application No. 180756/1988. Namely, an accumulator is provided in parallel with each of the hydraulic engaging elements, and an oil discharge control valve is provided in an oil discharge passage that is connected to the hydraulic engaging element to be disengaged. The pressure decrease in the hydraulic engaging element to be disengaged is buffered or cushioned by the accumulator that is parallelly connected thereto and causes a simultaneous engagement to take place. Once the hydraulic oil pressure in the hydraulic engaging element to be engaged has increased to a certain degree, the oil discharge control valve is opened to rapidly decrease the hydraulic oil pressure in the hydraulic engaging element to be disengaged, thereby preventing an excessive simultaneous engagement.
If one accumulator must be provided for each of the hydraulic engaging elements as in the above-described conventional example, there is a disadvantage in that a large number of accumulators are required and consequently that the control apparatus becomes large in size.
By the way, in the above-described conventional example, there is interposed in the first-speed transmission train a one-way clutch which allows for overrunning of the output side. The first-speed transmission train is thus established by engaging the hydraulic engaging element for the first-speed transmission train and, while this hydraulic engaging element is kept engaged, other hydraulic engaging elements are respectively engaged to thereby establish the other transmission trains. As described above, in the transmission provided with a hydraulic engaging element that is kept engaged in a plurality of transmission trains, the accumulator for that particular hydraulic engaging element will always be kept in a condition of pressure accumulation.
Taking note of this point, the present invention has an object of providing a control apparatus in which the pressure decrease characteristics at the time of disengagement of other hydraulic engaging elements can be controlled by the accumulator for the hydraulic engaging element that is always kept engaged, thereby making it possible to decrease the number of accumulators to be used.